Lithium salts which have conventionally been used in electrolytes etc. in lithium secondary batteries are solids having a highly dissociative chemical structure and do not show ion conductivity by themselves. Accordingly, the lithium salts have conventionally been used after dissolution in a suitable solvent in order to confer ion conductivity thereon.
Solvents used in dissolving lithium salts are generally non-aqueous solvents from the viewpoint of suppressing a reaction with an electrode active material. General non-aqueous solvents are combustible, and thus use of such non-aqueous solvents is preferably avoided in lithium secondary batteries widely usable for consumer application.
For withholding the use of non-aqueous solvents, a lithium ionic liquid having lithium ion conductivity and high dissociation property (self-dissociation property) by the lithium salt itself has been proposed. For example, Patent Document 1 discloses a lithium ionic liquid having a lithium cation and a quaternary anion having the XIII group element (B, Al, Ga, In, Ti) as the central element. However, there is the case where the lithium ionic liquid disclosed in Patent Document 1 cannot attain sufficiently excellent ion conductivity.
Patent Document 2 discloses a lithium ionic liquid represented by LiAlXn(OY)4-n wherein “X” is an electron-withdrawing substituent and “Y” is an oligoether group. However, the dissociation property of the lithium ionic liquid disclosed in Patent Document 2 is not high, and depending on the type of the electron-withdrawing group “X”, its viscosity may increase, and as a result, oligoether groups involved in conduction of lithium cations are hardly contacted with one another, thus failing to attain sufficiently excellent ion conductivity in some cases.
Patent Document 3 discloses a non-aqueous electrolyte for electrochemical element including a fluorine compound and an ambient temperature molten salt consisting of an onium cation and a non-aluminate anion. The addition of LiBF3(CF3), LiBF3(C2F5) and LiBF3(C3F7) etc., as lithium salts is also disclosed. However, the non-aqueous electrolyte for electrochemical element disclosed in Patent Document 3 includes various ions to make the system complex so that the movement of lithium cations may be inhibited.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2004-307481    Patent Document 2: JP-A No. 2003-146941    Patent Document 3: JP-A No. 2005-229103